Yesterday, I launched quakelive as usual, and my sensitivity was totally wrong.
It was very high compared to what I usually play with, and felt like there was some accel.
Of course I didn't change anything.

None of the above worked.
So, as I couldn't find any solution, I tried to lower my QL sens
to find my sens again. I found out a value quite close, but that wasn't as smooth as it used to be.

But I noticed something very strange, if I tried to put back my original QL sens, and put something like cl_mouseaccel -0.8, then the resulting sens was pretty close to what I used to play with.
So I was like, wow, do I play with negative acceleration ? Do I really need it ? Oo

As none of the settings suited me, I tried something different.
I installed the mouse drivers.
Driver sets default DPI to 800.

In QL, with 2.1, it was quite fast.
So I decided to use m_cpi parameters and set it to 800.
Of course the sensitivity went really slow, so I had to set it up to 12 or something like that, but I didn't really like the feeling of it.

I went back in the driver, and set DPI to 1600.
Back in QL I put m_cpi to 1600, and had ofc to increase the sens.
I put it up to 24 (!!!), and ended up with a sensitivity that suits me, even better than the original one (rofl).

But 24 is a really high value, I'm quite surprised.
Do you, or any player you know use such value ?
Is 1600 DPI too much ? Is quake able to be precise with sens @ 24 ? Should I lower m_cpi even if the mouse runs @ 1600 ?

Sorry for the long post.
This sensitivity thing is driving me mad. It happened a few times, but usually I just have to relaunch quakelive and it's back to normal.

I have been struggling quite a lot and would really appreciate any ideas :]

When configuring mouse sensitivity and acceleration, the physical meaning of the values you enter depends on factors outside of the game itself, namely the mouse's CPI (counters per inch, also referred to as DPI by most hardware manufacturers) and anything which manipulates the mouse input before it gets to the game (e.g. a driver-level sensitivity setting). A result of this is that one player cannot directly compare his in-game settings with another player - the external factors must also be taken into account for such a comparison to be meaningful. Changing from one mouse to another can also be a pain. In the past, players would perform a physical measurement with a ruler, or use a calculation to determine values which are meaningful (e.g. sensitivity in terms of cm/360). Although this procedure isn't too bad when talking about the base sensitivity, for more complex settings like acceleration, things can get rather messy.

In this update, a new cvar has been added (m_cpi) to solve this problem. By setting m_cpi to the effective CPI of your mouse, the input code can perform scaling in meaningful `natural' units (cm, degrees, seconds), rather than the arbitrary hardware-dependent units of `counts'. The default value is 0, which turns off the feature.

If two players are both using their correct values of m_cpi, they can compare their mouse settings directly from the cvars - this applies to both the base sensitivity and ALL acceleration cvars. For example, the sensitivity will be in units of degrees per cm, and cl_mouseAccelOffset will be a mouse speed in cm/s. This also means that if you are using m_cpi and you change to another mouse, m_cpi is the *only* cvar you have to change.

In order to set m_cpi correctly, it should take into account any driver-level sensitivity setting. For example, you have a 1000cpi mouse with a driver setting of x0.6 - you would set m_cpi to 600, not 1000. I refer to this as the `effective CPI'. If you do not have any driver settings, then just set m_cpi to the cpi of your mouse. Note that the value quoted in your mouse's documentation may differ very slightly in practice depending on the mouse surface, additional mouse feet, etc.

This feature is designed to be used with m_yaw at its default value of 0.022 (using another value will still function, but will no longer benefit from using the natural scale). You can still use m_pitch to provide your vertical axis with a different scale factor.

Old to New Conversion

In order to convert your existing sens and accel settings to the new scheme with m_cpi, there are some existing calculators out there that will do the conversion. If you want to do this manually, you can use the following formula for conversion from old to new.

its enough to use cvar_restart to get quake to default.
You probably had some other setting enabled or different feel on different day. Sometimes it can feel high or low. I pick up a ruler and measure 360 to make sure.

That's called sens-bug in QL and is a well known "feature" of the game. Just get used to it, it appears randomly on some PCs. Noone knows the real cause of it and the QL-Devs can't be arsed/don't have the manpower to fix it. Some people experience it others don't, I've never had that problem with it since I've started playing QL (wich was back in 2009), others got it recently or some time ago.